Aregu et al. Environ Syst Res (2018) 7:10 https://doi.org/10.1186/s40068-018-0112-2

RESEARCH Open Access Identifcation of two low‑cost and locally available flter media ( and scoria) for removal of hazardous pollutants from tannery wastewater Mekonnen Birhanie Aregu*, Seyoum Leta Asfaw and Mohammed Mazharuddin Khan

Abstract Background: Tannery wastewater contains the most hazardous pollutants. Therefore, identifying potentially efcient, low-cost and locally available flter media as an adsorbent for the treatment of tannery wastewater is critical. The aim of this study is to identify and assess the ability of identifed adsorbents and compare their efciency. The volcanic rocks of pumice and scoria were collected from the rift valley area of Oromia region, Ethiopia and their chemical char- acteristics were determined using X-ray fuorescence analysis. Batch mode experimental study design was carried out. The rocks were crushed and efective size was determined by using a standard sieve. The composite tannery waste- water was collected from Dire tannery, Addis Ababa, Ethiopia and treated with pumice and scoria. Two adsorption kinetics and isotherm models were conducted to predict the removal mechanism and capacity of the adsorbents on the reduction of ­NO3–N, ­PO4–P and ­[Cr]T from tannery wastewater. Analysis of wastewater samples was done before and after diferent retention time. R statistical software and Originlab pro 2017 was run for data analysis and graphing.

Results: The untreated tannery wastewater revealed that the mean concentration of BOD­ 5, COD, TSS, orthophos- phate, ammonium, nitrite, nitrate, sulfde, sulfate and chromium were beyond the permissible limits. Nitrate removal efciency of scoria and pumice were 99 and 95% respectively at retention time of 72 h. Phosphate removal was better by scoria on the frst 24 and 48 h. The efciency of pumice to remove sulfate was 83–84%, whereas scoria shows 75–77%. In the frst 24 and 48 h retention time, pumice and scoria achieved 76 and 71% in chromium reduction respectively. Conclusion: This study revealed that both scoria and pumice have a potential capacity to treat tannery wastewa- ter. Conversely comparing the average efciency to reduce hazardous pollutants scoria showed better results than pumice. Keywords: Tannery wastewater, Pumice, Scoria, Filter media, Removal efciency

Background hazardous pollutants (Shen 1999). Tannery wastewa- Industrial wastes are usually generated from diferent ter contains the most hazardous pollutants of industry. industrial processes, as a result the amount and toxicity Major problems caused by tannery wastewater contain- of waste released from industrial activities varies with ing heavy metals, nutrients, toxic chemicals, chloride, the type of industrial processes. Among all the industrial lime with high dissolved and suspended salts, and other wastewater tannery wastewaters are the most source of pollutants. In developing countries, many industrial units are operating in a small and medium scale. Tese indus- trial units can generate a considerable pollution load *Correspondence: [email protected] by discharging untreated or partially treated efuents Centre for Environmental Sciences, College of Natural Science, Addis directly into the nearby environment (Asfaw 2014). Ababa University, Addis Ababa, Ethiopia

© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Aregu et al. Environ Syst Res (2018) 7:10 Page 2 of 14

In Ethiopia currently, there are more than 30 tannery Adsorption on the other hand is the process of accu- industries in operation. Among them the majority found mulation of a substance on the surface of another sub- in the Oromia region, especially Modjo town and around stance. When a solid surface is exposed to a water or six established in the capital city Addis Ababa. Tese tan- wastewater, molecules from the solution phase accumu- neries have 153,650 sheep and goat skin soaking capac- late or concentrate at the solid surface. It is recognized ity and 9725 cowhides soaking capacity per day together as one of the most efective wastewater purifcation tech- they also employ 4577 persons (UNIDO 2012). nique used in several industries. Te basic principle of Te total wastewater discharge estimation from tanner- adsorption is mass transfer and adsorption of a molecule ies is about 400 million m3/year in Ethiopia. About 90% from a liquid into a solid surface. It will happen if the pol- of world leather production use chrome-tanning pro- lutant has low solubility in the wastewater, greater afnity cesses rather than vegetable tanning. In Chrome tanning for the substrate than wastewater and the combination of process tanneries utilize chromium in the form of basic the two (Rao et al. 2007). chromium-sulphate for hide stabilization against micro- Depending on the type of force of attractions between bial degradation and provision of fexibility of the leather. the pollutant and adsorbent, the adsorption process can In chrome tanning process about 60–80% of chromium be divided into two types. Physical adsorption or chemi- reacts with the hides and about 20–40% of the chromium cal adsorption. If the force of attraction between pollut- amount remaining in the solid and liquid wastes (Rezic ant and adsorbent is weak that is a Vanderwaal force of and Zeiner 2008). attraction, the process is called physical adsorption or Tanneries generate wastewater in the range of 30–35 L/ commonly known as physisorption. Physical adsorption kg of skin or hide processed with variable pH and high takes place with the formation of multilayer of pollutant concentrations of suspended solids, BOD and COD. on the adsorbent. But if the force of attraction between Major problems are due to wastewater containing heavy pollutant and adsorbent is chemical forces of attraction metals, toxic chemicals, chloride, lime with high dis- or chemical bond, the process is called chemical adsorp- solved and suspended salts and other pollutants (Durai tion or chemisorption. Chemisorption takes place with et al. 2011). Hexavalent chromium from tannery waste- the formation of a single layer of pollutant on the adsor- water is one of the major concerns of environmental pol- bent. In general, there are factors infuencing adsorption lution. Tis is due to discharge of tannery wastewater in such as surface area, nature of the pollutant, hydrogen large quantities without or with partial treatment (Lof- ion concentration (pH) of the wastewater, temperature, rano et al. 2008). mixed solutes and nature of adsorbent (Dabrowski 2001). Developing countries face numerous challenges related Adsorption has been identifed as one of the most to preserving the environment from industrial wastewa- promising mechanism for removal of dissolved heavy ter pollution. Like many other developing countries, Ethi- metal fractions and nutrients from wastewater. Although opia also grieves from environmental pollution problems commercial adsorbents are available for use in adsorp- of wastewater particularly industrial wastewater. Tis tion, they are very expensive, resulting in various new issue seems to be a subject which has not yet received low-cost adsorbents being studied by researchers. (Babel adequate attention during the development of industries. and Kurniawan 2003), reviewed the technical feasibility Terefore, there is a need to develop efcient and low- of various low-cost adsorbents for heavy metals removal cost wastewater treatment technologies for the removal from wastewater and concluded that the use of low-cost of heavy metals and other pollutants. Among these tech- adsorbents may contribute to the sustainability of the nologies, adsorption and fltration is a user-friendly tech- surrounding environment and ofer promising benefts nique for this purpose. for commercial purpose in the future. In fltration technique, wastewater containing sus- Terefore, identifying potentially efcient, low-cost pended matter is added to the top of the flter medium and locally available flter media as an adsorbent is criti- as the wastewater flters through the porous medium, cal for proper practice of environmental management by the suspended matter in the wastewater is removed by a tanning industries. On the other hand ordinary sand for diferent of mechanisms. Tese mechanisms are strain- flter media is costly because of construction-expansion ing, sedimentation, impaction, interception, adhesion, in the country, not available readily and not efcient in adsorption, focculation and biological degradation the removal of hazardous pollutants by adsorption hence especially for organic removals on the top of the flter there is a need to substitute pumice and scoria instead of medium. A natural characteristics of flter medium are sand fltration. important in the pollutant removal performance. Some Pumice is a light, porous volcanic that forms dur- of them are efective size, size distribution, slope, density ing explosive eruptions (Fig. 1). It resembles a sponge and porosity (Boller and Kavanaugh 1995). as it consists of a network of gas bubbles frozen amidst Aregu et al. Environ Syst Res (2018) 7:10 Page 3 of 14

Fig. 1 Typical pumice: photograph Mekonnen Birhanie March/2016, Ethiopia

fragile volcanic glass and minerals. All types of treatment processes. Pumice was used as a flter medium (, , dacite, and rhyolite) will form pumice, and as a support material for microbial growth in water however it is most commonly formed from rhyolite. Dur- and wastewater treatment (Farizoglu et al. 2003). ing an explosive eruption, volcanic gases dissolved in the Te other is scoria generally denser than liquid portion of magma also expand rapidly to create a the pumice. Scoria is somewhat porous material with foam or froth; in the case of pumice, the liquid part of the high surface area and strength with density larger than froth quickly solidifes to glass around the glass bubbles. one. Scoria is an excellent medium which holds water in Pumice is considered a glass because it has no crystal its pores and allow air circulation to the root zone of the structure. Like many of the materials considered in this plant. Both pumice and scoria are widely available in the report, it is an aluminosilicate (Akbal et al. 2000). Rift valley area of Ethiopia. Studies have shown using substrate rich in iron (Fe), Scoria is bomb-sized, generally vesicular pyroclastic aluminum (Al) or calcium (Ca) concentrations enhances rock with basaltic composition, which is reddish brown phosphate removal in experimental subsurface wetlands to black in color and is of low density (Fig. 2). It has beyond that which can be achieved by using native soils been used in several industrial applications, such as the (Arias et al. 2001). It was believed that sedimentation of manufacturing of a lightweight concrete mixture, a heat- particulate phosphorus and sorption of soluble phospho- ing-insulating material, low-cost fllers in paints, and Sor- rus (onto the pumice) were responsible, and that the high bents (Moufti et al. 2000; Alemayehu and Lennartz 2009). concentrations of iron (18.2%), aluminum (13.7%), cal- Scoria is abundant in many places worldwide including cium (12.7%) and magnesium (7.3%) in the pumice were Central America, Southeast Asia (Vietnam, etc.), East the source of this high sorption ability. Te low specifc Africa (Ethiopia, Kenya, etc.), and Europe (Greece, Italy, gravity and high porosity of pumice make it important Spain, Turkey, etc.) (Kwon et al. 2005; Alemayehu and for a number of applications in water and wastewater Lennartz 2009).

Fig. 2 Typical scoria: photograph Mekonnen Birhanie March/2016, Ethiopia Aregu et al. Environ Syst Res (2018) 7:10 Page 4 of 14

Sorption of contaminants onto scoria mainly takes on the analysis the efective size (ES) ­(d10) of each media place at the outside surface at the initial stage. Changes of was 1.5–4.5 and the uniformity coefcient (UC) ­(d60/d10) ionic composition during sorption experiments suggest is 3.5–4. After grading the flter materials were washed that cation exchange is likely the dominant mechanism with tap water and dry in sunlight for 1 week. of heavy metals sorption onto scoria, while considerable Two fltration tanks were made of metal sheet, each As(III) removal by scoria is explained by specifc sorption with the following dimension, 60 cm height and 28 cm of the neutral As(III) species and electrical adsorption of diameter and also were ftted with a half-inch an outlet negatively charged As(V) species via As oxidation onto tap (faucet) 5 cm above from the bottom of each tank. hematite (Kwon et al. 2010). Te experimental investi- Te fltration tanks were installed at College of natu- gation conducted demonstrates that the scoria is able to ral and computational science, Addis Ababa University. concurrently reduce concentrations of heavy metals and After installation the flter media were flled in the fltra- arsenic in aqueous solutions. Kwon et al. (2010), recom- tion tank 10 cm depth with 10–25 mm grain size, drain- mend that scoria can be used as an economic and ef- age layer at the bottom, 30 cm depth flter layer with a cient Sorbent to treat contaminated water with heavy grain size of 1.5–4.5 mm in the middle and the distribu- metals. tion layer (fat coarse gravel) was added 5 cm depth at the Taking into account the growth of industrialization top of the flter media to protect erosion of flter’s top lay- in Ethiopia and the expected demand for industrial ers, then it is ready for sample tannery wastewater fltra- wastewater management, low-cost, appropriate and tion (Fig. 4). eco-friendly approaches will play a critical role in the development of future wastewater treatment technol- Wastewater sample collection and fltration ogy in the country. In this practical approach, this work Te composite sample tannery wastewater was collected deal with the principles of adsorption and fltration for from Dire tannery and transported to Addis Ababa Uni- the removal of hazardous pollutants from tannery waste- versity in 40 L plastic ‘Jerican’for each flter tank. Te on- water by identifying these two volcanic ashes (scoria and site measurement of the physicochemical parameters was pumice) as a flter media instead of conventional sand. undertaken. Te collected raw tannery wastewater was added to the two fltration tank at the time and a sample Methods also transported to Addis Ababa EPA water and waste- Study area and period water analysis laboratory and Ethiopian Construction Tis study has been conducted in Addis Ababa University Design and Supervision Works Corporation Research by transporting sample wastewater from Dire tanning Laboratory (ECDSWCRL) for the raw tannery wastewa- industry located at Kolfe Keranio sub city Addis Ababa ter characterization. Ethiopia from May to August 2016. Filtrated sample collection and laboratory analysis Study design Te physicochemical analysis of wastewater samples Batch mode comparative experimental study design has has been done before and after the treatment with the been carried out to assess the ability and determine the two flter media, using standard methods (APHA 2005). efciency of scoria and pumice flter media on removal Optimum operating treatment time was determined for of hazardous pollutants by the treatment of industrial maximum removal of these impurities by running the wastewater, the case of tannery wastewater fltration. experiment for 24, 48 and 72 h, respectively. Filtrated samples were taken by 2 L plastic bottle after each fxed Materials, experimental, design and set‑up establishments retention time that is over 24, 48, and 72 h and trans- Te volcanic rocks were collected from volcanic cones in ported to Addis Ababa EPA water and wastewater analy- the rift valley area of Oromia region, East Shewa Zone, sis laboratory and ECDSWCRL after taking each sample. Ethiopia, (pumice collected from the area lies in between: Te analytical parameters were pH, DO, ­BOD5, COD, 8°28′36″N and 39°14′29″E; scoria collected from the area TSS, ammonium N, nitrite N, nitrate N, phosphate, lies in between: 8°35′47″N and 39°08′45″E) approximately sulfde, sulfate and chromium. On-site measurement of 50–100 km East of Addis Ababa (Fig. 3). the wastewater like temperature, pH and DO were car- Te rocks are local volcanic rocks with various chemi- ried out at the site in the tannery environmental quality cal and mineralogical structure and transported to Addis control laboratory using a portable pH meter (Wagtech Ababa University. Te chemical characteristics of both International N374, M128/03IM, USA) and DO meter flter media were determined by XRF analysis (Table 1). (Hach P/N HQ30d, Loveland. CO, USA). Tese flter media were crushed and graded. Te efec- COD, ammonium–nitrogen, nitrite–nitrogen, nitrate– tive size was determined by using a standard sieve. Based nitrogen, phosphate, sulfde and sulfate were measured Aregu et al. Environ Syst Res (2018) 7:10 Page 5 of 14

330000.000000 400000.000000 470000.000000 540000.000000 610000.000000 680000.000000 Legend 0 0 .00000 .00000 0 Collection Site of Scoria 0

108000 Collection Site of Pumice 108000

0 Location of Modjo Town 0 .00000 .00000 0 0 East Showa Zone 102000 102000 0 0 .00000 .00000 960000 960000 0 0 .00000 .00000 900000 900000 0 0 .00000 .00000 840000 840000

330000.000000 400000.000000 470000.000000 540000.000000 610000.000000 680000.000000 Fig. 3 Collection site of pumice and scoria

by using a spectrophotometer (Hach model DR/3900 using BOD sensor and inductive stirring system AQUA portable spectrophotometer, Germany) according to LYTIC model type ET618-4 and fame atomic absorp- Hach instructions. BOD­ 5 and total Cr were analyzed tion spectrophotometer (AAS), (model AAS NOUA-400, Aregu et al. Environ Syst Res (2018) 7:10 Page 6 of 14

Table 1 Physical and chemical characteristics of scoria At equilibrium, qt qe and Ct Ce; therefore the amount and pumice = = of adsorbed chromium, qe, was calculated by Eq. (2). Chemical composition Percent weight V (C − C ) q = 0 e Scoria Pumice e W (2)

SiO 52.46 64.92 2 where C0, Ct and Ce are the initial concentration, con- Al2O3 18.14 10.82 centration at any time and equilibrium concentrations of Fe2O3 5.40 4.62 chromium in the wastewater (g/L), respectively, V is the CaO 9.40 5.74 volume of the wastewater (L), and W is the mass of the K2O 0.20 4.26 fler medium (kg) (Elmorsi 2011). Na2O 3.28 4.92 Te removal efciency of the flter medium for the MgO 7.44 0.92 selected parameters including chromium were calculated MnO 0.12 0.20 as Eq. (3). P O 0.36 0.14 2 5 (C − Ct ) Removal% = 0 × 100 TiO2 0.41 0.15 (3) C0 H2O 1.12 0.85 LOI 2.08 3.67 where C0 is the parameter concentration in the untreated pH 7.81 7.53 wastewater and Ct is the parameter concentration in the aPhysical properties, particle size 0.075–0.425 mm treated wastewater at the fnal hydraulic retention time t. = Porosity (%) 36 73 3 Particle density ­(gcm− ) 2.96 2.33 Statistical data analysis 2 1 Specifc surface area (BET) ­(m g− ) 2.49 3.5 Mean and standard deviations were calculated to esti- 1 Cation exchange capacity (CEC), mequiv. 100 g− 0.09 0.84 mate the concentration of each parameter of the samples. Te hypothesis has been tested by student t test using R LOI loss on ignition statistical software: R version 3.2.2 (2015-08-14), Plat- a Alemayehu et al. (2011) form: x86_64-w64-mingw32/x64 (64-bit) to determine whether an observed diference between the means of the groups is statistically signifcant or not, based on the treatment efciency of the flter materials. OriginPro 15cm 2017 64-bit was used also for graphing and plotting for Raw TWW 5cm Raw TWW adsorption kinetics and isotherms model analysis. Flat & Coarse gravel Flat & Coarse gravel

30cm Data quality management

Fine Pumice Fine Scoria To assure quality of the data by minimizing the errors the following measures had been undertaken: Apparatuses 10cm were calibrated; expiry date of reagents had been checked before starting the real analysis and standard control also Coarse Pumice Coarse Scoria prepared. Each test had been triplicated.

abResult and discussion Fig. 4 Schematic layout of the flter tanks a components of pumice, Physicochemical characteristics of Dire tannery wastewater b components of scoria Te raw wastewater was taken from Dire tannery around Kolfe Keranio Sub city Asko area, Addis Ababa, Ethiopia and transported to the Addis Ababa EPA laboratory and Germany) respectively. Total suspended solids (TSS) ECDSWCRL for physicochemical analysis. Based on this were determined according to the standard methods for investigation the mean concentrations of selected phys- the examination of water and wastewater gravimetric icochemical parameters were presented in Table 2. method (APHA 2005). Tis study revealed that the mean concentration of Te amount of chromium from the wastewater ­BOD5, COD and TSS were 1081 ± 160, 12,913 ± 6875 adsorbed onto the flter medium at any time, q , was cal- t and 2426 ± 515.20 mg/L respectively (Table 2). Tis culated as Eq. (1). result is basically similar to diferent studies in Ethio- V (C − C ) pia with slight diferences for diferent parameters, for q = 0 t t W (1) example a study done at Modjo tannery indicated that Aregu et al. Environ Syst Res (2018) 7:10 Page 7 of 14

Table 2 Characteristics of Dire tannery wastewater sulfde and sulfate were 417 ± 131 and 1307 ± 224 mg/L SNo Parameter Concentration (mg/L) Range respectively. In this case the amount of sulfde found except pH and T° in this study wastewater was more or less equivalent to 1 pH 9.1 3.10 6.5–12.50 study done by Islam et al. (2014) that is 380 ± 50 mg/L. ± 2 T° (°C) 20.6 2.30 19–22 Sugasini and Rajagopal (2015), also characterize the tan- ± 3 BOD 1081 160 924–1243 nery wastewater based on their result the concentration 5 ± 4 COD 12,913 6875 8046–21,025 of sulfate was 1517 mg/L which is almost parallel to this ± 5 TSS 2426 515.20 1849–2840 investigation. In terms of chromium concentration, Dire ± tannery comprised 35.7 90 mg/L is similar to other dif- 6 NH –N 314 60 259–378 ± 4 ± ferent results presented from various tannery wastewa- 7 NO –N 1.7 0.30 1.4–1.99 2 ± ters in Ethiopia for example a study done by Leta et al. 8 NO –N 124 13 110–135 3 ± (2003) indicates 32.2 5.70 mg/L. On the other hand two 9 PO –P 168 74 112–252 ± 4 ± more study results showed, the chromium concentration 10 Sulfde 417 131 334–568 ± lied to be in the ranges of this investigation 28–45 mg/L 11 Sulfate 1307 224 1118–1555 ± as in Table 2 (Tadesse and Seyoum 2015; Asaye 2009). 12 Total chromium 35.7 90 28–45 ± Even though Wastewater of each tannery process con- sists of varying pH and temperature values, this study results (9.1 ± 3.10 and 20.6 ± 2.34 °C) respectively were the mean concentration of COD was laid between 7950 analogous to diferent studies. Likewise a large variation and 15,240 mg/L with the mean of 11,123 ± 563.90 mg/L exists in values of physicochemical parameters in gen- (Leta et al. 2003). Another study also undertaken with eral like ­BOD5, COD, TSS, phosphate, sulfde, sulfate, same tannery wastewater showed that the mean con- etc. in every tannery wastewater characteristics, this may centration of ­BOD5 was 1054 ± 448 mg/L (Tadesse be because of diferent tanning process, methods, tech- and Seyoum 2015). But the concentration of total sus- nology and raw material utilization by various tanning pended solid was found to range from 1849 to 2840 mg/L industries. (Table 2) this result is a bit greater than some studies, for instance a study done in India indicated 1244 mg/L Adsorption kinetics for the pollutant removal process (Mandal et al. 2010). of tannery wastewater treatment using pumice and scoria Nutrients like orthophosphate, ammonium, nitrite flter medium and nitrate concentration of Dire tannery were charac- From the experimental data three selected pollutant terized in this study, the result revealed that 168 ± 74, parameters were tested to ft into diferent kinetic mod- 314 ± 60, 1.7 ± 0.30, 124 ± 13 mg/L respectively. Tis els for the adsorption rate, the removal mechanism and result is comparable to a study done by Sivakumar et al. process and predict information about the interaction (2015) which indicates the concentration of nitrate in between these two naval adsorbents (flter medium) and untreated tannery efuent as 116 mg/L. Te result of those selected three pollutants ­(NO3–N, ­PO4–P and ammonium is in the range of the results done at Bahir ­[Cr]T) in the tannery wastewater (Martins et al. 2013). In Dar tannery wastewater characterization (96–420 mg/L) this study, two models were used, these are the pseudo- (Wosnie and Wondie 2014). According to Sugasini and frst-order (Lagergren 1898) and the pseudo-second- Rajagopal (2015), the nitrite concentration of untreated order kinetics model (Ho and McKay 1999). tannery wastewater was 1.3 mg/L almost parallel to this study fnding which accounts 1.7 ± 0.30 mg/L (Table 2). Pseudo‑frst‑order equation Whereas the concentration of orthophosphate in this Pseudo-frst-order equation was given frst by Lagergren study was 168 ± 74 mg/L, this result shows that the con- (1898) to determine the rate constant of adsorption pro- centration of phosphate in Dire tannery wastewater is cess as Eq. (4). higher than other study results done previously to char- k acterize another tannery wastewaters. Te variation may − = − 1 log (qe qt ) log qe t (4) be due to the utilization of phosphorus containing chem- 2.303 icals for diferent purposes and tanning activities in Dire where qe and qt are the amounts of the pollutants tannery. adsorbed (g/kg) at equilibrium and at time t (h), respec- Te total suspended solids in Dire tannery found to −1 tively and k1 is the rate constant of adsorption ­(h ). be 2426 mg/L this result is more or less similar with the Values of k1 and qe were calculated from the slope and results of tannery wastewater analyzed by Banuraman the intercept of the plots of log (qe qt) versus t for and Meikandaan (2013). Te concentration level of both − NO­ 3–N, ­PO4–P and [Cr]­ T adsorption from the tannery Aregu et al. Environ Syst Res (2018) 7:10 Page 8 of 14

wastewater onto pumice and scoria shown in Figs. 5a and kinetic model except [Cr]­ T does not ft only onto scoria 6a respectively. substrate. Te results in Tables 3 and 4 show that the values of correlation coefcients (R2) were high for all parameters Pseudo‑second‑order equation adsorption on to both adsorbents except chromium Pseudo-second-order equation formulated from the adsorption onto scoria (Table 4) and the experimental adsorption equilibrium (Ho and McKay 1999). Te equa- qe value agree with the calculated value with very slight tion can be expressed in Eq. (5): diference for all parameters except ­PO4–P and ­[Cr]T d q = − 2 adsorption onto scoria (Tables 3, 4). Terefore, this k2(qe qt ) (5) dt study revealed that the adsorption of the three pollut- ants onto both adsorbents ft with the pseudo-frst-order

Fig. 5 a Pseudo-frst-order and b pseudo-second-order kinetics for the adsorption of NO­ 3–N, ­PO4–P and ­[Cr]T removal onto pumice

Fig. 6 a Pseudo-frst-order kinetics. b Pseudo-second-order kinetics for the adsorption of NO­ 3–N, ­PO4–P and ­[Cr]T removal onto scoria Aregu et al. Environ Syst Res (2018) 7:10 Page 9 of 14

Table 3 Kinetic model parameters for the adsorption of ­NO –N, PO­ –P and ­[Cr] onto the pumice (pH 9.1, T° 21 °C) 3 4 T = = Pollutants Initial conc. (g/L) Pseudo-frst-order kinetic Pseudo-second-order kinetic

1 2 1 1 2 qe, exp. (g/kg) qe, cal. (g/kg) K1 (h­ − ) R qe, cal. (g/kg) K2 (kg g− h− ) R

NO3–N 0.124 1.160 1.140 0.020 0.9969 1.302 0.024 0.6419

PO4–P 0.168 1.080 1.010 0.030 0.9711 1.189 0.049 0.8604

[Cr]T 0.0357 0.268 0.267 0.035 0.9999 0.276 0.415 0.8938

Table 4 Kinetic parameters for the adsorption of ­NO –N, PO­ –P and ­[Cr] onto the scoria (pH 9.1, T° 21 °C) 3 4 T = = Pollutants Initial conc. (g/L) Pseudo-frst-order kinetic Pseudo-second-order kinetic

1 2 1 1 2 qe, exp. (g/kg) qe, cal. (g/kg) K1 (h­ − ) R qe, cal. (g/kg) K2 (kg g− h− ) R

NO3–N 0.124 0.250 0.249 0.032 0.9951 0.282 0.19 0.8535

PO4–P 0.168 0.210 0.149 0.053 0.8769 0.220 0.87 0.9806

[Cr]T 0.0357 0.051 0.025 0.039 0.4690 0.539 2.61 0.9294

where qe and qt are the sorption capacity (g/kg) of pol- Te ­NO3–N is strongly agree with Pseudo frst-order lutants at equilibrium and at time t respectively and k2 kinetic model by considering both correlation coef- 2 is the rate constant for pseudo-second order sorption fcients (R ) and experimental and calculated qe value, −1 −1 (kg g h ). For the boundary conditions t = 0 to t = t but ­PO4–P agree with second-order kinetics in terms of and qt = 0 to qt = qt, the above equation has been inte- both kinetic parameters using scoria adsorbent. Similarly grated and linearized to make Eq. (6). ­[Cr]T removal process onto scoria substrate also agrees 2 t 1 1 with the second-order model based on the value of R , = + × t 2 which is comparable to a study done in South Korea and qt k2 × q qe  (6) e Nigeria on the adsorption of Cr(VI) ion onto diferent  −1 −1 where k2 (kg g h ) is the adsorption rate constant of natural adsorbents (Ali et al. 2016; Owalude and Tella pseudo-second-order equation. Te value of qe and k2 2016). However, in this kinetic model for ­[Cr]T onto the can be obtained from the slope and the intercept of the same adsorbent (scoria), the experimental qe is not in- plot of (t/qt) versus t respectively for those selected pol- line with the calculated one (Table 4). lutants adsorption from the tannery wastewater onto pumice Fig. 5b and scoria Fig. 6b. Adsorption equilibrium isotherm modeling of chromium Te results in Figs. 5b and 6b show linear plots with removal from tannery wastewater 2 very high values of R on the adsorption of PO­ 4–P and In this study, two diferent adsorption isotherm models, 2 ­[Cr]T onto scoria. But the value R for NO­ 3–N adsorp- namely Langmuir and Freundlich were used to evaluate tion onto pumice was low. In addition the result showed the afnity of the two studied adsorbents (flter medium) that, good agreement between experimental and calcu- for the removal of ­[Cr]T from the real tannery wastewater. lated values of qe in the adsorption kinetics of ­PO4–P and Langmuir isotherm ­[Cr]T onto pumice adsorbent (Tables 3, 4). Terefore, the adsorption of ­PO4–P and [Cr]­ T onto the scoria represents Te Langmuir isotherm model assumes that a monolayer a good ft with pseudo-second-order kinetics. In this adsorption at specifc homogenous sites such as ­[Cr]T in case the adsorption of ­PO4–P and [Cr]­ T onto the scoria real tannery wastewater is adsorbed on the adsorbent process suspected to be chemisorption Alemayehu et al. surface in this specifc case pumice and scoria. Equa- (2011). tion (7) is the Langmuir expression. 2 In terms of R , Table 3 clearly shows, all the three pol- Ce 1 Ce lutant removal process in the pumice containing fl- = + qe KLqm qm (7) ter tank strongly agree with pseudo-frst-order kinetic model than second-order. A result of chromium adsorp- where qm (the maximum capacity of adsorption, g/kg) tion kinetics and equilibrium study done by Pandey et al. and KL (a constant related to the afnity of the binding (2010) showed the same trend with this kinetic study sites, L/kg) are the Langmuir isotherm constants. Both qm fndings. and KL can be determined from the slope and intercept of Aregu et al. Environ Syst Res (2018) 7:10 Page 10 of 14

the plot Ce/qe versus Ce which is the linear form of Lang- the adsorption of ­[Cr]T onto pumice follows Langmuir muir equation that gives a straight line showed in Figs. 7a equation and also it shows that pumice has a maximum and 8a. adsorption capacity of ­[Cr]T than scoria (Tables 5, 6). Te Langmuir isotherm constants of [Cr]­ T onto pum- Even though the previous researchers used laboratory ice and scoria substrates were displayed in Tables 5 scale test with synthetic solution unlike this study, Ale- and 6 respectively. Te high value of correlation coef- mayehu et al. (2011) obtained similar result with this 2 cients (R ) = 0.9464 in pumice substrate indicated mini- fnding. mal deviation from the ftted equation showing that

Fig. 7 a Langmuir and b Freundlich isotherms for the adsorption of ­[Cr]T onto pumice

Fig. 8 a Langmuir and b Freundlich isotherms for the adsorption of ­[Cr]T onto scoria Aregu et al. Environ Syst Res (2018) 7:10 Page 11 of 14

Table 5 Langmuir and Freundlich isotherm constants fltration method are more efective than chemical and for the adsorption of ­[Cr] onto the pumice (pH 9.1, T = biological processes, but at the same time more expen- T° 21 °C) = sive. So alternative adsorbent must be investigated. In Langmuir constants Freundlich constants this study two volcanic rocks pumice and scoria were 2 2 identifed and tested for their ability in removal of haz- qm (g/kg) KL (L/kg) R KF (L/kg) 1/n R ardous pollutants and compared the efciency for the 0.071 135.75 0.9464 250.5 0.897 0.9174 − treatment of tannery wastewater by adsorption and fltra- tion techniques. Based on this fnding the pollutant removal efciency Table 6 Langmuir and Freundlich isotherm constants of pumice is better than scoria to remove BOD­ 5 at each for the adsorption of ­[Cr] onto the scoria (pH 9.1, retention time, but in terms of COD removal reveres T = T° 21 °C) result was obtained. In general the two substrates were = Langmuir constants Freundlich constants poor to remove both ­BOD5 and COD this is obvious that the organic matters cannot be removed by adsorption 2 2 qm (g/kg) KL (L/kg) R KF (L/kg) 1/n R some may be removed biologically at the top of the fl- 0.016 120.652 0.9088 997.7 0.872 0.7857 ter medium. Better removal efciency has been seen by − Scoria to remove TSS, the minimum removal efciency (65%) was achieved by pumice at RT = 24 h and the maxi- mum removal efciency (84%) was obtained by scoria Freundlich isotherm at RT = 72 h. Tis better removal efciency was seen Te Freundlich exponential equation presumes that the because most of the TSS may be removed by the straining adsorption process takes place on a heterogeneous sur- mechanism of fltration in both flter materials (Table 7). face. Te linear form of Freundlich equation is given in In nutrient removal potential, scoria shows better ef- Eq. (8). ciency than pumice for example nitrate removal ef- 1 ciency of scoria and pumice were 99 and 95% respectively = + log qe log KF log Ce (8) n at RT = 72 h. Similarly the phosphate removal also better by scoria on the frst 24 and 48 h, but at RT = 72 h pumice where KF (L/kg) is an indicator of the multilayer adsorp- removed 66% and scoria removed 63% only. Tis study tion capacity and 1/n is the adsorption intensity and result is in line with study done in Jimma University Ethi- indicates both the relative distribution of energy and the opia using an aqueous solution running over 24 h contact heterogeneity of the adsorbent sites. Figures 7b and 8b time with similar flter media (Birhane et al. 2014) with a representing the linear plot of log qe versus log Ce at con- slight diference. Tis diference may be due to the phos- stant temperature. Te value of KF and 1/n (Tables 5, 6) phate concentration, experimental setup and the type of were determined from the intercept and the slope of the wastewater used for the test between the two studies. plot of log qe versus log Ce respectively. In another study that was tested using four flter mate- Linear regression is the widely used approach in evalu- rials. Te removal of phosphate ranged from 35 to 41% ating the ft between experimental data and the various for calcite, 59 to 100% for , 49 to 100% for sand, isotherm models, this can be determined from the value 2 2 and 73 to 100% for iron flings (Reddy et al. 2013). From of (R ) (Ncibi 2008). In this study, R (0.9174) of Freun- the indicated four flter materials the result of this paper dlich isotherm on the adsorption of [Cr]­ T onto pumice can be compared in the range of the result obtained from is greater than the value of R2 (0.7857) onto scoria sub- 2 iron flings. Tat means pumice is similar adsorption strate. On the other hand the value of R is greater in characteristics with iron flings. Te removal of nitrate the Langmuir isotherm model than Freundlich in both and phosphate may be mainly attributed to adsorption, substrates. Terefore adsorption of [Cr]­ T onto both sub- ion exchange and precipitation. Te reverse trend was strates ft with the Langmuir and Freundlich isotherm shown to remove sulfate and chromium that indicate 2˃ model with R 0.75. pumice had a better efciency than scoria in all given retention time (Table 7; Fig. 9). Comparison of the efciency of pumice and scoria Since industrial efuents like tannery wastewater substrate for tannery wastewater treatment containing sulfde and sulfate are toxic to aquatic envi- Wastewater from the leather industry is known to be ronment, it is essential to reduce them and bring the heavily contaminated with inorganic and organic pollut- discharge levels of these species to below the toxic limit. ants. Treatment processes for such type of high strength In this investigation the sulfate removal potential of wastewaters is challenging. In this case adsorption and pumice was greater than scoria the efciency of pumice Aregu et al. Environ Syst Res (2018) 7:10 Page 12 of 14

Table 7 Comparison of scoria and pumice flter media efciency in tannery wastewater treatment Parameters % removal at RT 24 h % removal at RT 48 h % removal at RT 72 h = = = Pumice Scoria Pumice Scoria Pumice Scoria

BOD5 56 52 57 55 62 59 COD 27 48 30 50 45 54 TSS 65 75 68 83 70 84

NH4–N 41 32 28 27 23 6

NO2–N 88 98 95 98 97 97

NO3–N 40 51 56 80 95 99

PO4–P 41 57 51 60 66 63 Sulfde 70 73 71 75 72 77 Sulfate 84 77 83 76 83 75 Total chromium 76 71 76 71 70 69 Average 58.8 63.4 61.5 67.5 68.3 68.3

to remove sulfate was from 83 to 84% whereas scoria mainly to sulfde removal in the fltration tank. In a study shows from 75 to 77%. In general both flter mediums carried out to investigate the efect of chemical modifca- were efective to remove sulfde and sulfate from tannery tion method on sulfate removal efciency of adsorbents, wastewater. the removal of sulfate ion using Fe-modifed carbon resi- Te better removal performance of pumice may be due was notably higher compare with unmodifed car- achieved based on the good sulfate adsorption nature of bon residue and commercially available activated carbon the pumice and its chemical composition. On the other (Runtti et al. 2016). hand metal ions from the flter material may react with Batch experimental laboratory scale studies were dissolved sulfde ions to form metal sulfdes as colloidal undertaken in a diferent world on the potential of pum- suspension, which were coagulated and precipitated and ice to reduce the concentration of heavy metals including fnally fltered out, this mechanism may be contributed chromium from aqueous solution in the laboratory. But

100

90

) 80 (%

l 70 va 60 mo 50 ge Re 40 ta

en 30 rc

Pe 20

10

0 PUMICE SCORIA PUMICE SCORIA PUMICE SCORIA 24 HOURS48 HOURS72 HOURS

NitratePhosphate SulfateT. Chromium

Fig. 9 Comparison of scoria and pumice on the removal efciency of nutrients, sulfate and chromium Aregu et al. Environ Syst Res (2018) 7:10 Page 13 of 14

in this study the chromium removal efciency of pumice use of locally available low cost adsorbents may contrib- and scoria was done from real tannery wastewater. ute to the low technology solution for sustainable waste- In this study pumice had better in chromium reduction water management. Any interested company can use potential than scoria. In the frst 24 and 48 h retention these substrates as a flter medium in the fltration bed or time pumice and scoria achieved 76 and 71% in chro- substitute by sand for available fltration bed for waste- mium reduction respectively (Table 7). A study done by water treatment. (Alemayehu et al. 2011), showed similar result that was pumice had a better result to remove chromium than sco- Abbreviations ria. Te maximum adsorption yield, 77% for scoria and APHA: American Public Health Association; BOD: biological oxygen demand; 80% for pumice, was obtained at low pH in that study. COD: chemical oxygen demand; DO: dissolved oxygen; ECDSWCRL: Ethiopian Construction Design and Supervision Works Corporation Research Labora- Tis chromium removal potential diference between the tory; EPA: environmental protection authority; ES: efective size; RT: retention two substrates may be due to the diference in chemical time; TSS: total suspended solids; TWW​: tannery wastewater; UC: uniformity composition (Table 7; Fig. 9). coefcient; UNIDO: United Nations Industrial Development Organization; XRF: X-ray fuorescence. Considering all the selected tannery wastewater parameters for this study, the average treatment ef- Authors’ contributions ciency of pumice and scoria were 58.8 and 63.4% in All authors have made an essential intellectual contribution to this study. MBA designed the study, conducted the experiments, Collected, analyzed RT = 24 h, 61.5 and 67.5% at RT = 48 h respectively, and and interpreted the data and wrote the manuscript. SLA involved on the equivalent efciency was obtained at the third retention study design, supervised the experiment, provided comments and sugges- time that is over 72 h both flter materials showed 68.3%. tion for the whole work. MMK supervised the work, drafting and revising the primary manuscript, edited the manuscript, provided pertinent comments Even though there were diferences in diferent types of and suggestion on the manuscript. All authors read and approved the fnal parameter reduction, the average tannery wastewater manuscript. treatment efciency of scoria was greater than pumice Authors’ information at the frst 24 and 48 h, but it is not statistically signif- Mekonnen Birhanie Aregu is a Ph.D. Scholar in environmental pollution and cant at 95% confdence interval, p = 0.3 and 0.2 respec- sanitation, Center for Environmental Sciences, College of Natural Science, tively. Equivalent percentage removal (68.3%) was shown Addis Ababa University and lecturer at School of Public Health, Dilla University. He has given Public health, Environmental Health Science and Technology in both substrates at RT = 72 h, but in the overall result, courses and also contribute in community services and problem solving Scoria had shown greater efciency than pumice but it applied research activities and published several articles in the internationally is not statistically signifcant at 95% confdence interval, peer reviewed journals. He is also a senior consultant in environmental health, waste management and emission control. p = 0.2. Seyoum Leta Asfaw (Ph.D.) is an Associate Professor of environmental pol- lution and sanitation stream, Center for Environmental Sciences, Addis Ababa Conclusion University. He has given various courses in the stream. He is also supervising and monitoring several Ph.D.s and M.Sc. students in the areas of environmen- Based on the investigation the following major conclu- tal biotechnology, wastewater treatment and water quality studies, waste sions have been drawn: to energy (resource recovery and climate change mitigation and adaptation Dire tannery wastewater characteristics were very high studies), bioremediation, phytoremediation, microbial ecological studies, environmental sanitation. He has published several peer reviewed papers in strength wastewater with diferent hazardous pollutants. diferent international reputable journals. Currently, he is also executive Direc- Te applied methodology provides an efcient and feasi- tor for Horn of Africa Regional Environmental Centre and Network (HOARECN). ble approach for removal of pollutants. He has more than 20 years of experience in environmental science and technology studies and management. He had also been a regional program Pumice had good potential to reduce pollutants from manager for the Bio-resources innovation network for Eastern Africa develop- tannery wastewater, especially nutrients, sulfate and ment. He has also been the principal investigator for a number of research chromium but it is poor in terms of reduction of organic projects such as “Development of innovative technologies for the sustainable treatment of high strength wastewater in East Africa”, a regional research pollutants. Scoria also showed promising result in the program involving Ethiopia, Kenya, Tanzania and Uganda funded by Sida. He reduction of nutrient, sulfate and chromium from tan- developed an innovative, integrated pilot technology for the treatment of nery wastewater. Except sulfate and chromium the other agro-process wastewater, generating biogas, biofertilizer, and clean water at Modjo Tannery, Addis Ababa, Ethiopia. wastewater parameter reduction was directly propor- Professor Mohammed Mazharuddin Khan (Ph.D.) is a professor of tional to the retention time of the fltration system. Environmental Science at Center for Environmental Sciences, Addis Ababa More or less both substrates have a potential to treat University. He is the CHARLES DARWIN GOLD MEDAL was conferred on the 27th day of June, 2009 towards the contribution made in the feld of Life Sci- high strength industrial wastewater like tannery efuent ence by International Society for Ecological Communication at Vinoba Bhave by using diferent techniques including fltration and can university, Hazaribagh, Jharkhand, India. He has given several courses like be an alternative to sand fltration. However, when we Botany, Biotechnology, Environmental sciences, Ecology, Microbiology, and Environmental Microbiology at postgraduate and undergraduate level. He is compare the average efciency to reduce those selected also a founder of diferent organizations, moreover he is a member of more wastewater parameters scoria showed better results than eight international professional associations and societies and served in than pumice, even though pumice has better potential to diferent positions. Professor Khan has published several internationally peer reviewed papers. reduce chromium and sulfate than scoria. Terefore the Aregu et al. Environ Syst Res (2018) 7:10 Page 14 of 14

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